[Reland] Correct fixed-position recording for Slimming Paint

Source/core/layout/LayoutBoxModelObject.cpp

Index: Source/core/layout/LayoutBoxModelObject.cpp
diff --git a/Source/core/layout/LayoutBoxModelObject.cpp b/Source/core/layout/LayoutBoxModelObject.cpp
index fae0ee81a9a7ff82697301c4d7295a6bd2e9ee64..484b46260853f73340798b89476681de432c2ffe 100644
--- a/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/Source/core/layout/LayoutBoxModelObject.cpp
@@ -224,6 +224,17 @@ void LayoutBoxModelObject::styleDidChange(StyleDifference diff, const ComputedSt
             setChildNeedsLayout();
     }
 
+    // Fixed-position is painted using transform. In the case that the object
+    // gets the same layout after changing position property, although no
+    // re-raster (rect-based invalidation) is needed, display items should
+    // still update their paint offset.
+    if (RuntimeEnabledFeatures::slimmingPaintEnabled() && oldStyle) {
+        bool newStyleIsFixedPosition = style()->position() == FixedPosition;
+        bool oldStyleIsFixedPosition = oldStyle->position() == FixedPosition;
+        if (newStyleIsFixedPosition != oldStyleIsFixedPosition)
+            invalidateDisplayItemClientForNonCompositingDescendants();
+    }
+
     if (FrameView *frameView = view()->frameView()) {
         bool newStyleIsViewportConstained = style()->hasViewportConstrainedPosition();
         bool oldStyleIsViewportConstrained = oldStyle && oldStyle->hasViewportConstrainedPosition();